The invention relates to a fastening element for fastening insulating sheets or panels and if necessary additional sealing sheets to a solid base, said element comprising a screw and a large-surface metal washer, wherein the screw has a drilling point, a screw shank provided with a thread at least over part of its length and a screw head, and the washer has a preferably central hole for introducing the screw shank, wherein the washer can be preassembled at a distance from the screw head in the region of the screw shank such that it is secured against inadvertent axial shifting and wherein a tubular extension aligned axially parallel to the screw shank to be inserted adjoins the central hole in the washer, as well as to a method for preassembly of a large-surface washer in the threaded region of the screw shank of a screw so as to secure against inadvertent axial shifting.
Fastening elements of the type mentioned in the foregoing are known in many alternative embodiments (for example, U.S. Pat. No. 4,781,503 or EP A 0283184). In these embodiments, it is ensured that a large-surface washer is held directly in the region of the screw head or in a thread-free shank section disposed directly under the screw head so as to be secured against axial shifting. Thereby a certain degree of walking safety is supposedly achieved to the effect that if, in the use of fastening elements on roof surfaces, for example, persons walk on the insulating sheets or the sealing sheets laid thereover, the screw head is supposedly prevented from pushing through the sealing sheet to the outside due to such loading. In one known embodiment there is provided as the large-surface washer a plastic disk, which is braced against a circumferential collar formed on the screw shank. In another known embodiment there is provided a metal washer in which bracing takes place at the end of the threaded section adjacent to the screw head.
There are also known a fastening element and a device designed for driving this fastening element (U.S. Pat. No. 4,809,568), wherein this known fastening element comprises a screw and a washer made of plastic. The washer has in the central region, or in other words in the region of a hole provided to accommodate the screw shank, an elongated hub, so that the wall of the through hole can bear against the thread of the screw over a relatively long section. By dimensioning the hole in the washer to appropriate close tolerances, it can be ensured that the screw is pushed onto the thread in the manner of a fit, and in this position remains preassembled until finally set.
In practice, however, it is not universally possible to use plastic washers, especially in those sections of a roof where sealing is subsequently completed by a flame process. It is then imperative to use metal, large-surface washers. In order nevertheless to be able to achieve preassembly between washer and screw, and specifically at correspondingly large distance from the screw head, attempts have been made to inject in a relatively large central hole a plastic part, which has sufficient thickness viewed in axial direction that it can then bear against the screw over a certain threaded region, thus permitting preassembly in the form of a fit. It was soon realized that such an additional arrangement of a central plastic part in a metal washer is associated with enormous costs, which are unthinkable for a mass-produced article.
To be able to drive such fastening elements with a practical hand-held tool, it must be ensured that the large-surface washer is fixed on the screw shank at appropriately large distance from the screw head, since axial alignment of the screw is achieved while it is being driven, on the one hand by the fact that the screw head is engaged in the tool and on the other hand by the fact that the screw is guided at appropriate distance in the hole of the washer, so that at least two-point guidance is achieved until the screw point penetrates into the solid base. For this purpose it has already been proposed (German Patent Application DE A 4239339) that the hole in the washer must be made smaller than the threaded outside diameter on the screw shank, or at least as large as the core diameter of the threaded region. Thereby the washer can be preassembled in the threaded region of the screw shank at a distance from the screw head, so as to be secured against axial shifting. During preassembly, the screw is driven sufficiently far into the washer that the washer is disposed in the correct axial position relative to the screw head. It is precisely in screws with a relatively long thread, however, that the process of driving the screw practically through the disk is extremely time-consuming.
This disadvantage is also found in a fastening element of another known design (WO96/11311). After all, the tubular extension provided therein has an internal thread, which engages with the external thread of the screw. In addition to this, an adhesive can also be introduced. It is also possible, however, to fix the preassembled configuration exclusively by means of an adhesive. In one of the embodiments there are provided for this purpose a tubular extension without internal thread, a close fit between extension and screw, and an adhesive which fills the extension. The first-cited embodiment, in which screw and extension are in mutual threaded engagement, has the further disadvantage that the internal thread of the extension must be made by thread grooving or thread cuffing, which represents a major expense in a mass-produced article such as the fastening element in question here. Furthermore, the extension must have large material thickness to ensure that thread grooving or thread cutting is possible at all. Finally, this thread-grooving or thread-cuffing operation must be accomplished in a separate, additional working sequence, and only thereafter will the washer be united with the screw, by driving the latter sufficiently far into the washer. All this means increased time requirements, which must also be added to the time requirement for driving the screw into the washer. Moreover, the thread of the screw and the thread of the washer must lie within certain tolerances to ensure that the thread surfaces of surface-treated screws and washers will not be damaged during the driving process.
Accordingly, it is the object of the invention to provide a fastening element of the type indicated in the introduction, in which optimal preassembly can be achieved in a short time and with little working effort.